What is a Flue Gas Source Heat Pumps? – Boiler Guide

What is a Flue Gas Source Heat Pump?

A flue‑gas‑source heat pump (FGSHP) pulls the low‑grade heat—usually below 150 °C—from boiler or industrial exhaust and “upgrades” it via a refrigerant cycle (evaporate → compress → condense → expand). By dropping exhaust temperatures to around 15–30 °C and reclaiming both sensible and latent heat, it delivers hot water or space heating at high temperatures while squeezing out every bit of energy from the fuel—efficiencies can exceed 110% on an LHV basis.

How Does It Work? The Thermodynamic Breakthrough

FGSHP leverages a ​​multi-stage heat recovery process​​ anchored in vapor-compression refrigeration principles:

1. Evaporation: The cold refrigerant flows through the evaporator, where it boils at around 30–60 °C by drawing heat from the flue gases.

2. Compression: The resulting vapour is sucked into the compressor, which cranks up its pressure—and its temperature climbs into the 70–90 °C range.

3. Condensation: Hot, high‑pressure vapour then moves through the condenser, dumping its heat into the water circuit (for things like space heating or domestic hot water).

4. Expansion: After condensing, the liquid refrigerant passes an expansion valve that drops its pressure and temperature, sending it back to the evaporator to repeat the process.

​​Critical Innovation​​: By using flue gas as the heat source instead of ambient air, FGSHP overcomes the ​​winter efficiency collapse​​ plaguing air-source heat pumps. The stable flue gas temperature (even at -30°C ambient) ensures year-round COP > 4.5.

Three Industrial-Grade FGSHP Architectures

1. ​​Direct-Expansion FGSHP​​

• ​​Mechanism​​: Refrigerant directly exchanges heat with flue gas in a corrosion-resistant ​​cast silicon-aluminum evaporator​​. This achieves ​​20–30°C exhaust temperatures​​ and >110% system efficiency.
• ​​Advantages​​:
  • Eliminates “white plumes” by condensing 91% of vapor.
  • Reduces NOx by 10%+ via acid dissolution in condensate.
• ​​Deployment​​: Ideal for industrial boilers and district heating.
  • Case: Beijing residential project cut gas use by ​​360,000 m³/year​​ and recovered 3,000 tons of condensate.

2. ​​Screw-Drive FGSHP (Indirect)​​

• ​​Design​​: Uses intermediate water loops to transfer heat from flue gas to a screw-compressor heat pump. Exhaust temperatures drop to ​​<15°C​​ with COP ≤ 7.0.
• ​​Advantages​​:
  • Delivers 65–75°C output water—ideal for process heating.
  • Avoids high-pressure steam equipment certifications.
• ​​Deployment​​: Hospitals, schools, and commercial complexes.
  • Case: Beijing hospital reduced gas consumption by ​​21.5%​​ with COP 4.73.

3. ​​Centrifugal FGSHP (Indirect)​​

• ​​Scale​​: Integrates plate heat exchangers with centrifugal compressors for ​​40–80 MW capacity​​. Exhausts at ≤30°C with primary-energy COP of 4–7.
• ​​Advantages​​:
  • Supports heating for 800,000 m² buildings.
  • Annual CO₂ reductions up to 160,000 tons.
• ​​Deployment​​: Gas-fired power plants and city-scale heating networks.

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Why FGSHP Outperforms Traditional Systems

​​Added ROI​​:

• ​​60% lower heating costs​​ vs. gas boilers.
• Payback period: ​​1–4 years​​ (varies by energy prices).

FAQs: Addressing Industry Skepticism

Can FGSHP handle acidic condensate corrosion?​

Yes. Evaporators use ​​silicon-aluminum alloy​​ with 8–10× the thermal conductivity and corrosion resistance of stainless steel. Surface roughness promotes dropwise condensation, preventing scaling.

Does flue gas variability affect performance?​

Variable-frequency compressors and adaptive controls maintain stable COP across 30–150°C flue gas fluctuations.

Is 15°C exhaust practically achievable?​

 Absolutely. Beijing projects consistently achieve 15–23°C exhausts, validated by third-party audits.

The Future Is Clear: Efficiency Beyond Condensation

Flue Gas Source Heat Pumps redefine energy recovery—transforming exhaust from a waste stream into a strategic asset. With ​​>26% fuel savings​​, ​​zero visible emissions​​, and ​​ROI < 2 years​​, this technology isn’t just incremental improvement; it’s the next standard for industrial decarbonization.

​​Ready to maximize your energy assets?​​ Contact our engineers for a site assessment.